Micellar exit rates

Micellar Exit Rates. In micellar solubilization, the dominant factors governing the exit and reentry rates of solubilizates are largely unknown. The exit rates for naphthalene, biphenyl, and 1-methylnaphthalene from ionic micelles are >5 X 104 s exit rates for anthracene and pyrene are reported as... 

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

Table 5.2. Exit rate constants ki for solubilizates in micellar systems. (From Ref.77 )...

The inhomogeneity of the micellar aggregate also affords assisted spin trapping and the exploitation of magnetic field effects on the charge separated ion pairs [48]. Optical modulation spectroscopy can be used, for example, to follow the decay of radicals formed in homogeneous solution and in SDS micelles. Enhancements of a factor of about 50 in the lifetimes and the steady state concentrations of the radical were observed in the micelle, and a kinetic analysis led to a value of 2 x 103 s 1 for the exit rate constant from the micelle [49]. 

Figure 2 presents a schematic of one scenario in which nonionic surfactant may assist biomineralization. In this situation micellar nonionic surfactant has solubilized HOC from soil. As microorganisms deplete aqueous-phase HOC via mineralization, the micelle releases HOC to solution. HOC exit rates from micelles may be significantly faster than HOC desorption rates from soil, and this condition thereby potentially enhances the availability of HOC to the microorganism. Other researchers (25, 28) suggested that surfactants may make HOCs more available for microbial attack in soil by decreasing the interfacial tension between the compound and water. 

The extent of phenanthrene transfer far exceeds the amount that may b< attributed to dissociation of micelles due to the movement of surfactant mon omers to hexane (52). Thus, both reported PAH exit rates from surfactan micelles and screening studies on phase transfer of micellar phenanthrene from water to hexane suggest very rapid exit rates compared to rates o mineralization. 

The entry/exit rate constants of ketones with SDS micelles was smdied by using either a micellar quencher such as y-methylvalerophenone or nitrite ion as an aqueous quencher (Table 17) [193,194]. Using a micellar quencher, the values of kp were determined by employing Eq. (27), which incorporates the fraction of micelles containing a probe molecule... 

In conventional reversed phase HPLC, differences in the physicochemical interactions of the eluate with the mobile phase and the stationary phase determine their partition coefficients and, hence, their capacity factor, k. In reversed-phase systems containing cyclodextrins in the mobile phase, eluates may form complexes based not only on hydrophobicity but on size as well, making these systems more complex. If 1 1 stoichiometry is involved, the primary association equilibrium, generally recognized to be of considerable importance in micellar chromatography, can be applied (11-13). The formation constant, Kf, of the inclusion complex is defined as the ratio of the entrance and exit rate constants between the solute and the cyclodextrin. Addition of organic modifiers, such as methanol, into the cyclodextrin aqueous mobile phase should alter the kinetic and thermodynamic characteristics of the system. This would alter the Kf values by modifying the entrance and exit rate constants which determine the quality of the separation. 

In the early studies, the micellar reduced efficiency was attributed to a diminished rate constant for solute exit from the micellar aggregates [5]. If the solute stays inside the micelles that move with the mobile phase for a long time, it interacts with less stationary phase. It "sees" a shorter column. Assuming the solute entrance in the micelle is diffusion controlled and equivalent for all solutes, the exit rate constant is inversely proportional to the constant. The solutes with the highest affinity for the micellar phase should be the least retained. Actually, they are the most retained solutes which shows that the solute affinity for the micelles (Pwm) are strongly correlated to the affinity for the surfactant covered stationary phase (Pws)- These combined effects with the hydrophobic solutes show that they are the ones with the poorer efficiency [5]. The reduced solute exit rate from the micelle produces a decreased solute diffusion coefficient in the mobile phase, D. ... 

The 1983 work of the Dorsey research group established the use of 3% v/v 1-propanol in micellar phases to reduce the MLC efficiency problem [3]. It is now confirmed that the addition of alcohol to micellar phases (i) increases the rate of the solute mass-transfer between the micelles and the aqueous phase by increasing the solute micelle exit rate constant, (ii) increases the solute mass transfer kinetics between the stationary phase and the aqueous phase by decreasing the stationary phase viscosity and the amount of adsorbed surfactant. The problem of alcohol addition to micellar phases is that kinetics enhancements cannot be dissociated from thermodynamics changes. The efficiencies increase and the retention times decrease. A hybrid alcohol-micelle mobile phase has necessarily a higher solvent strength than a purely aqueous phase [34]. It was shown that alcohols were changing the micelles and the stationary phase in a comparable manner [26] as noted on the Pws and parallel variations in Table 6.4. 

TABLE III Solubilizate Entry and Exit Rate Constants in Micellar SDS ... 

Pseudophase models work for several reasons (i) Reactions in association coUoids can be carried out under conditions of dynamic equilibrium. Thus the totality of the interfacial regions of all the aggregates in micelle, microemulsion, or vesicle solutions, and the totalities of their oil and water regions can be modeled as single interfacial, oil, and water reaction volumes of uniform properties with a separate rate constant for the reaction in each volume. Scheme 4. (ii) The requirement of dynamic equilibrium is met because the rate constants for diffusion of ions and molecules in association colloid solutions are near the diffusion-controlled limit. For example, the entrance and exit rate constants in micellar solutions in Table 1 are orders of magnitude faster than the example rate constants for thermal bimolecular reactions in micellar solutions in Table 4. Many additional examples are compiled in reviews. (iii) Measured rate constants for spontaneous reactions and... 

Case la. Reactions occur completely in the micellar phase (the exit rate constant for out lAo ) interphase exchange by product C is much faster than the reactions and decay of excited molecules (exit rate constant for C /c"ut kout lAo )- The stationary concentration [C] of the product is equal to its equilibrium concentration in the micellar phase, which is determined by its overall concentration [C] in the solution, its distribution coefficient q" and the volume concentration of the micellar phase X ... 

The photoprotolytic reactions in micellar solutions show the usual values of the deuteration isotope effect [122] which gives evidence that the photoprotolytic dissociation is not controlled by the exit of the excited molecules from the micelles. The product of the dissociation A can be formed within a micelle initially and then can leave the micelle. The increase of the number of the carbon atoms in surfactant molecule causes an increase of the exit rate constant. 

With the development of new dosage form technology in which control of drug release is achieved, it is conceivable that micellar systems will find some place because of the ability of the micellar phase to alter the transport properties of solubilized drug molecules. One can envisage the deliberate addition of surfactants to drug reservoirs to control the exit rate of drugs from polymeric devices. This will be explained in Chapter 7. 

Electron transfer rates between adrenaline and related benzene diols and complexes of iron(III) with some substituted 1,10-phenanthrolines have been reported [67] in surfactant systems. In cationic systems the reactions take place in the aqueous phase and reaction rates are lower than they are in simple aqueous systems, but in anionic surfactant systems the reaction rates are enhanced, reactions probably taking place at the micellar interface. The rates of exit and entrance of aromatic compounds from and into micelles have recently been studied using phosphorescence decay measurements [68] exit rate constants of aromatic hydrocarbons are of the order of 10 to 10 s " S whereas values of 10 to 10 (moll ) s have been reported for intramicellar energy transfer processes. Release of aromatic phosphorescence probes from micelles followed by their deactivation in the aqueous phase is hence expected to be an important mode of deactivation of the triplet state [69]. Kinetic schemes for triplets that are partitioned between aqueous and micellar phases are considered for the cases of single occupancy and double occupancy of the micellar units. 

For protolytic photodissociation of hydroxyaromatic compounds we may expect that in anionic micelles the excited anions of hydroxyaromatic compounds having the same sign of charge will leave the micelles, and in the cationic micelles - hydrogen ions will leave micelles. Exit rate constants of excited anions of hydroxyaromatic compound from the micelles were determined using the non-solubilized fluorescence quenchers. Simultaneously, we have proved that it is in the micellar phase that protolytic dissociation does proceed and not as a result of the preliminary exit of excited molecules of hydroxyaromatic compounds from the micellar phase to an aqueous one. 

The surfactant exit rate constant from the aggregates present in the discontinuous (micellar) cubic phase formed by some surfactants at high concentration has been determined by NMR self-diffusion, The reported values of k compare well with those obtained in the case of micellar solutions. 

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